The characteristics of bioregulatory substances associated with embryogenesis and fetal development are under investigation. Placental and fetal tissues, taken from mice in the period from the 12th day of gestation through late term, were homogenized under acid and neutral conditions. Acid extracts of both types of tissue contained growth factors which elicited a mitogenic response in quiescent cells of mouse and rat origin when tested in a semi-micro assay procedure developed during this reporting period. The growth factor(s) was distinguishable from epidermal growth factor (EGF) in two ways: it was active in cell strains which were refractory to EGF; and it failed to compete with 125I-EGF for binding sites on A-431 human epidermoid carcinoma cells. Neutral extracts were similar in growth factor activity to acid extracts, but contained interferon-like antiviral activity as well.